This invention relates generally to liquid level gauging systems, and more particularly to systems wherein a voltage is applied to one of two conducting probes, with a corresponding potential being experienced by the other probe, such potential being influenced by the immersion of the probes in the liquid being gauged.
In the past, a number of such liquid level gauges have been proposed and produced. Most prior units employed probes consisting of two concentrically mounted conductive tubes which were mechanically secured and electrically insulated from one another by means of insulating spacers. Voltage was generally applied to the outer tube, with the inner tube being connected to a detector which sensed the potential assumed by it.
While such systems functioned quite well immediately after installation, there tended to be a gradual build up of debris, oil film, and other foreign material on the spacer members disposed between the tubes, especially after prolonged periods of use. This especially occurred where the spacing between the tubes was quite small, and where the spacers which were used were disposed in positions transverse to the flow of the liquid being gauged (such flow being in a direction parallel to the axes of the tubes and occurring in the space therebetween). In the event that the debris had conductive or dielectric characteristics which were somewhat different from that of the liquid being gauged, there would be introduced substantial errors in the readings by virtue of the short-circuiting effect of the debris on the electrodes. In addition, due to the fact that the flow between the conductive tubes was rather restricted, the likelihood of such debris being flushed out periodically was extremely remote. Accordingly, after a period of use it was usually necessary to remove the probe unit and forcibly flush out any foreign material which had become lodged between its electrodes.
Concentric-tube probe devices were generally open only at the bottom, and thus did not permit free flowing of the liquid being gauged into the space between the tubes. Such circumstances tended to introduce errors into the readings, since the nature of the liquid occupying the space between the electrodes at any particular time was not necessarily similar to that of the liquid occupying the remainder of the tank.
In prior devices employing spaced-apart metal electrodes or rods where there were insulating spacers or washers, a build-up of debris and other foreign matter associated with oil storage facilities occurred. Parallel electrodes without such spacers were fragile, and the spacing between the electrodes tended to change. In addition, many prior liquid gauges were expensive to produce in that they required complex assembly operations and adherence to close physical tolerances. For example, in the concentric-tube type of probe device, the spacing between the tubes had to be maintained to a close tolerance, which meant that multiple spacers were required. The time involved with producing such probes was frequently rather high, resulting in a product which was expensive to both produce and maintain.